A ring oscillator in which a plurality of inverters are arranged in series to obtain a signal oscillating at a desired frequency and in which an output of a final stage is fed back to a first stage is known. FIG. 8 shows a structure of a ring oscillator circuit as generally known. Five inverters INV1–INV5 are arranged in series, and an output of the final-stage inverter INV5 is fed back to the input of the first-stage INV1. In this example, the oscillating operation is sustained since there are an odd number of inverters. A ring oscillator of this type is also described in, for example, a patent document No. 1.
[Patent Document No. 1]
Japanese Laid-Open Patent Application No. 2001-85994 (entire text)